In a wireless cellular communications system which uses forward link (from a base station to a remote, e.g. mobile terminal) rate adaptation and fast scheduling for packet data services for which the packet data traffic is relatively insensitive to delay, it has been proposed to use fast cell/sector switching to replace or supplement soft handoff. For simplicity, the term fast cell switching and its abbreviation FCS are used herein to embrace fast cell or sector switching.
An advantage of FCS is that transmission takes place to each mobile terminal from only one cell (or sector) at any particular instant or time slot. This eliminates a need for scheduling coordination among multiple cells in dynamic packet data environments, and reduces both forward link power and inter-cell interference, thereby facilitating an increase in system capacity.
In FCS, a mobile terminal determines an optimum cell (or sector) within an active set via which it wishes to receive communications on the forward link (or downlink), and communicates this to the base station (BTS) through reverse link (or uplink) signalling. In order to determine an optimum cell or sector, the mobile terminal can average over several time slots the C/I (carrier-to-interference ratio) for each cell or sector, and can select the cell or sector having the best average C/I. The term C/I is used herein to embrace any channel quality estimate or indicator.
A disadvantage of this is that there is a delay between the selection of the optimum cell or sector and the actual switching to this cell or sector, this delay (referred to herein as an FCS delay) including a reverse link signalling delay for the FCS request from the mobile terminal, and a switching delay at the base stations. While prediction can be used to estimate the C/I to allow for the FCS delay, such prediction is unreliable in the event that the C/I varies rapidly, as is typically the case for a mobile terminal moving faster than a relatively low speed. Accordingly, in such cases FCS may provide an overall significant degradation of performance, rather than an improvement as is desired.
Accordingly, there is a need to facilitate implementing FCS in an effective manner which can avoid or substantially reduce this disadvantage.